A typical circuit board comprises a substrate having a pattern of copper conductive tracks formed onto it. The substrate can either be rigid, as in a printed circuit board (PCB), or flexible (the term "circuit board" will be used to cover both types). Active and passive electronic components are soldered onto the board via a set of pads arranged around the periphery of a component site. Each pad is the termination of a conductive copper track providing an input/output line to that component.
In one standard manufacturing process for PCBs the copper pads are protected from oxidation before the components are attached by coating the pads with a thin layer of solder (the tracks themselves are normally protected by a polyimide coverlay). The pads are first cleaned in a flux solution which comprises a mixture of acid cleaner and solvent and sometimes oil, and then immersed into a bath of molten solder. On withdrawal from the bath, solder only adheres to the exposed copper pads, but not to other regions of the PCB. It is important to ensure that an even coating of solder is applied, and one way of doing this is with a hot air solder levelling (HASL) machine, such as that described in GB 1457325. In a HASL machine, as the PCB is lifted out of the solder bath, it is blasted by jets of hot air from nearby nozzles which blow away excess solder. GB 1181421 describes an alternative method of removing excess solder by shaking or vibrating the circuit board after removal from the solder bath. In GB 1181421 the solder bath is covered by a thin layer of peanut oil.
As an alternative to immersion in a solder bath, it is also possible to use a solder wave process to coat the circuit board with solder. Another slightly different solder apply process is described in IBM Technical Disclosure Bulletin, 1982, Vol 24, No 11A, P5726 in which the level of solder in the tank is raised rather than the board lowered. The solder is covered by a thin layer of a glycerine solution to prevent oxidation of the solder, which overflows when the solder level is raised. The circuit board is vibrated while in the solder.
After the solder application stage, electronic components such as packaged silicon chips are mounted onto the PCB. The components usually have either a set of leads for soldering to the pads, or terminals on the component can be directly bonded onto the pads (such as with surface mount passive components). In either approach, additional solder is applied to the pads at this stage, typically by screening. This solder is used for bonding the components to the pads. As the size of the electronic components for such circuits has shrunk, so has the spacing of the pads surrounding them. In some cases adjacent pads are no more than 0.12 mm apart (approximately equal to the width of the pads themselves). Solder screening techniques are not at present capable of depositing such finely-spaced patterns of solder.
IBM Technical Disclosure Bulletin 1976 Vol 18 No 10 P3182-3183 describes one possible way around this problem, by applying solder to the components rather than the board. The components are pivoted through a solder bath and on removal from the bath, an oscillatory motion is produced using a spring stop to remove excess solder. Applying solder to the components rather than the board has the disadvantage that often the components have leads which need to be shaped, and this is hard to do once they have been coated in solder. Also, difficulties have been experienced in producing reliable joints, which are believed to be due to metallurgical problems at the bond between the lead and the solder.